On the lower intake just above the thermostat housing you will see two temp sensors screwed into the intake.
One will have 2 wires, that is the ECT sensor only used by the computer.
The other will have 1 wire, this is a temp sender only used by the dash board gauge.
The 1 wire sender is the one you want to test.
Engine cold, key on
temp gauge should be at C(cold)
remove the 1 wire from the sender, unplug connector
gauge should drop to below C
Get a piece of wire and push it into the connector make contact with the 1 wire
Ground the other end of that added wire, to engine or battery negative
Gauge should move all the way up to HOT
If above happens exactly this way then wire and gauge are fine, replace temp sender, make sure parts guy doesn't give you the 2 wire sensor, you want the 1 wire SENDER.
If gauge shows above C when engine is stone cold then yes, your wiring in the dash is the problem